The rotary drilling of oil and gas wells requires use of circulating fluids, usually identified as drilling muds. These muds are normally circulated in the wells by means of reciprocating pumps. The mud is pumped down the drill pipe and out into the well bore through small holes in the drill bit against the bottom and sides of the hole, and back up the well in the annular space between the drill pipe and walls of the wellbore, carrying with it drill cuttings and the like that are then removed before recirculation. While this mud performs a number of functions, including removing drill cuttings, keeping the bit cool, providing flotation to help support the weight of the drill pipe and casing and the like, one of the most important functions is to coat the wellbore surface to prevent caving in and prevent undesirable flow of fluids in or out of the wellbore, including drilling fluids, brine and the like. The properties of this coating are critical to future production and recovery.
Obviously, the properties of and the composition of these drilling mud formulations are complex and variable, depending on the conditions involved and the results desired or required. One of the most important properties of these drilling muds and other drilling fluids is their ability to resist filtration. To prevent formation fluids and the like from entering the wellbore hole, the hydrostatic pressure of the mud column must be greater than the pressure of the fluids in the pores of the formation. As a result, there is a tendency for the drilling fluids or muds to invade the permeable wellbore formations, that is, the liquid portions of the mud. This is referred to in the art as the filtrate. As a result, the mud solids are filtered out onto the walls of the wellbore hole and this often results in a coating or cake. The muds must be formulated so as to keep the deposited cake permeability as low as possible in order to maintain a stable borehole and minimize filtrate invasion of, and damage to, potentially productive strata and horizons in the production formation. Thus, formulation is critical and one major problem with drilling mud is this loss of filtrate, referred to as fluid loss.
Lignite, and its derivative, humic acid, have been widely used in drilling mud applications, often in association with amines. However, because of the poor dispersability of these materials in the oil phase in oil base muds, more effective fluid loss control agents are desired.
The fluid loss problem and its control differs with different drilling fluids, particularly drilling muds for rotary drilling. Two major types of drilling mud systems employed are often referred to as oil-in-water emulsions and water-in-oil emulsions, each type having its own particular advantages, problems, requirements and the like. In general, water-in-oil emulsions are preferred in many applications.
This invention is directed to oil based fluids or drilling muds, especially water in oil emulsions, that provide improved fluid loss as compared to known water in oil base drilling muds, particularly under conditions of high temperatures and pressures.